GRANITE PLUTONISM DURING CRUSTAL CHANNEL FLOW

Intermediate to felsic magmas (‘granites') have compositions that reflect the average composition of the continental crust and are intimately related to deformation and metamorphism in the continents. However, there is no general consensus on the mechanisms by which granitic plutons are emplaced. Common models for pluton emplacement call on magma driven ‘hydro'-fracture during dike and sill emplacement, the buoyant rise of magmas as diapirs with radial expansion, stoping, or into dilation regions controlled by shear zones. Here, we unite field observations of granitic plutons with geophysical observations of plateau and geodynamic models into a new model for pluton emplacement. The observations are consistent with a model for pluton emplacement where granitic plutons are emplaced as viscous ‘channel flows'. In this model sill like plutons can tunnel horizontally and vertically displacing plastically deforming country rock by shear across the pluton wall rock contacts. The driving forces for channel propagation include the buoyancy of the magma, pressure gradients produced by topography and regionally induced shearing during contraction. Many of the batholiths within the Coast plutionic complex of British Columbia have features consistent with crustal channel flow, including partially molten country rocks of low strength, gently dipping sills with shallow mineral lineations requiring largely lateral transport of melt, kinematics reversals which occur during emplacement and sills that were constructed during uplift and exhumation. This is an appealing model because pluton emplacement is linked directly to regional deformation, the magma can propagate into regions where all the stresses are compressive, negating the need for stress controlled tensile fracture, and magmas can intrude rocks of equal or less density because buoyancy forces are not the primary driver of channel propagation.